Former Defense Secretary Donald Rumsfeld, who oversaw the Pentagon during the early years of the wars in Afghanistan and Iraq, says he'll be voting for Donald Trump for president in November -- principally because the celebrity billionaire is "a known unknown."

Rumsfeld made his choice public Wednesday night during an interview on Fox TV's On the Record with Greta Van Susteren.

"On the Democrat side, we have a known known. On the Republican side, we have a recent entry who's a known unknown," he said, drawing on a phrase he made famous as defense secretary and used as the tile of a memoir in 2011.

Rumsfeld's backing of Trump drew a quick response from the progressive lobbying group VoteVets.org, which on Thursday said it is further evidence that the "neocons" who led the U.S. into war in Iraq are getting behind Trump.

Former Vice President Dick Cheney endorsed Trump in May.

"It's clear that neocon interventionists have found their puppet [in Trump]," retired Army Maj. Gen. Paul Eaton said. "From Donald Rumsfeld to Dick Cheney, neocons see in Trump a person who was for the war in Iraq, and continues to show an alarming willingness to launch a new war in the Middle East. It is clear that they feel that Trump is their perfect agent for a return to reckless Middle East ventures."

Both Cheney and Rumsfeld were signatories to the mission statement of the Project for the New American Century, a neoconservative think tank that called for shaping the broader world for U.S. interests, beefing up military spending and directly confronting foreign governments hostile to American interests.

Eaton, now senior adviser to VoteVets, served in Iraq in 2003 and 2004, commanding the Coalition Military Assistance Training Team responsible for training the Iraqi security forces. VoteVets.org is a veterans-based lobbying group that typically supports progressive candidates and causes.

Trump has often denied he supported the war in Iraq, and has made it a point to say he was against it. Eaton, citing arecent report by Politifact, said there is no evidence that supports Trump's contention that he spoke out against the invasion.

Eaton claims Trump showed his willingness to escalate in the Middle East last March when he said during a presidential debate that there is "no choice" but to send thousands more troops into the region to fight the Islamic State terrorists. "I'm hearing numbers of 20 [thousand] to 30,000. We have to knock them out fast," he said in the CNN debate.

Rumsfeld's comments on Fox News came hours after former National Security Adviser Brent Scowcroft, who had served in the administrations of Presidents Gerald Ford, George H.W. Bush and George W. Bush, endorsed Democrat and former Secretary of State Hillary Clinton for the presidency.

During the build-up to the war in Iraq, Scowcroft publicly opposed the invasion.

"[Clinton] brings deep expertise in international affairs and a sophisticated understanding of the world, which I believe are essential for the commander-in-chief," Scowcroft said in a statement Wednesday.

Last week, Richard Armitage, a Vietnam combat veteran who would later serve in the State Department under Ronald Reagan and the Defense Department during the Bush 41 administration, also endorsed Clinton.

"If Donald Trump is the nominee, I would vote for Hillary Clinton," Armitage told Politico. "He doesn't appear to be a Republican, he doesn't appear to want to learn about issues. So, I'm going to vote for Mrs. Clinton."

On Fox, Rumsfeld said he could not vote for Clinton because she lied to the families of Americans killed in the attacks in Benghazi, Libya, by saying the violence was prompted by a video of a Quran burning in Florida, rather than a planned terrorist attack.

He also cited Clinton's use of a private server for her official email, which continues to be investigated by the FBI. Rumsfeld said he believes reports that her private server gave her access to emails from departments that she should not have gotten.

"If she were a yeoman in the Navy or a sergeant in the Army or the Marine Corps or the Air Force, she would be prosecuted," Rumsfeld said.